CPU frequency issue: What is idma64.0, and what triggers its hyper-activity?

Following up on the old question CPU frequency always at the maximum, I have found that

sudo powertop

gives a process called idma64.0 with 300 to 4200 events/second when the CPU frequency is stuck at 3500 MHz, instead of slowing down. When the CPU frequency varies between 600 und 3500 MHz (as it should), the idma64.0 process can't be seen in powertop output. For comparison: When I have several Facebook tabs open in Firefox, powertop shows that /usr/lib/firefox/firefox is the most taxing process, clocking in at around 200 to 300 events/second.

Well, why is idma64.0 so active?

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